The subject invention relates to the treatment of copolymer films with a combination of crosslinking and surface treatments to obtain an elastomeric product which will retain its transparency upon relaxation after being stretched, be free of surface tack, and have surface characteristics such that the film displays acceptable heat sealability. Such a product may be used for packaging both food and nonfood items.
The development of copolymer films useful in the packaging industry has been an active area of experimentation for many years. Currently, there are generally two types of films used for packaging purposes. The first is heat-shrinkable film which, due to the temperature required to shrink-wrap such a film around a product, is undesirable for most fruit and vegetable packaging, as well as heat-sensitive nonfood packaging. The second is stretch film such as polyvinylidene chloride which lacks elastomeric properties and hence is not as desirable for packaging as are films which display more acceptable levels of elastic recovery. The object of this invention, therefore, is to devise a process whereby an elastomeric copolymer film can be treated so as to enhance its form stability, while remaining free of surface tack, transparent upon relaxation after being stretched and heat sealable.
It is known that crosslinking molecules of an elastomeric film such as ethylene/vinyl acetate copolymer (hereinafter EVA), containing about 35-50 percent copolymerized vinyl acetate by weight, by exposure to high energy ionizing radiation, results in a film with enhanced form stability, i.e., creep resistance. This crosslinking can be achieved by exposure to high energy radiation such as electrons, X-rays, gamma rays, etc., a dosage of such irradiation being measured in "rads" wherein one rad is the absorbed dose of ionizing radiation equal to an energy of 100 ergs per gram of irradiated material. A megarad (MR) is equal to one million rads. Crosslinked EVA displaying such form stability, however, is not satisfactory for stretchable film packaging due to excess surface tack which creates handling problems both in production windup and machine packaging applications.
The problem of surface tack associated with EVA has also been addressed by prior teachings. It is known that hydrolysis of EVA pellets render the pellets nonagglomerizable. It is also known that hydrolysis of the surface of EVA film removes the undesirable surface tack. However, hydrolysis of EVA referred to in prior teachings is carried out to such an extent that both elasticity and transparency after stretching would be lost from EVA in a film form.
Further, the combination of crosslinking EVA by irradiation techniques followed by hydrolysis with subsequent film formation has recently been addressed in U.S. Pat. No. 4,224,262 on the application of Baird et al. There the object of the invention was to produce a packaging film that would not be deleteriously affected by moisture and would, at the same time, serve as a barrier to oxygen permeation. To accomplish this objective, Baird et al. irradiated EVA copolymer containing 45 percent vinyl acetate by weight of the copolymer and subsequently carried out greater than 96 percent hydrolysis of the copolymer. A resin was recovered and pressed into a film of a crosslinked copolymer of an olefin and a vinyl alcohol. Because of the high degree of hydrolysis present throughout the thickness of the film, a total loss of elasticity is observed.